Simulated evolution process of core-shell microstructures

Abstract

The evolution process of core-shell microstructures formed in monotectic alloys under the space environment condition was investigated by the numerical simulation method. In order to account for the effect of surface segregation on phase separation, Model H was modified by introducing a surface free energy term into the total free energy of alloy droplet. Three Fe-Cu alloys were taken as simulated examples, which usually exhibit metastable phase separation in undercooled and microgravity states. It was revealed by the dynamic simulation process that the formation of core-shell microstructures depends mainly on surface segregation and Marangoni convection. The phase separation of Fe 65 Cu 35 alloy starts from a dispersed structure and gradually evolves into a triple-layer core-shell microstructure. Similarly, Fe 50 Cu 50 alloy experiences a structural evolution process of "bicontinuous phase → quadruple-layer core-shell → triple-layer core-shell", while the microstructures of Fe 35 Cu 65 alloy transfer from the dispersed structure into the final double-layer core-shell morphology. The Cu-rich phase always forms the outer layer because of surface segregation, whereas the internal microstructural evolution is controlled mainly by the Marangoni convection resulting from the temperature gradient.space environment, Fe-Cu alloy, phase separation, core-shell structure, numerical simulation Liquid phase separation of monotectic alloys has aroused great interest in the fields of materials physics and space science [1][2][3] . The earlier researches show that surface segregation and Marangoni convection resulting from the temperature gradient play an important role in the microstructural evolution under the space environment condition [2,4] . Due to the opacity of metals and the form requirements of the experiments, it is quite difficult to carry out theoretical investigation deeply. Therefore, the evolution process and the micro-mechanism are still poorly understood. Numerical simulation has become an important method to explore phase separation. However, it has been exclusively implemented to study the pattern formation of polymers and little attention has been

One-Step Fabrication of Al/Sn-Bi Core–Shell Spheres via Phase Separation

“…5b. Such a picture of composition rings has not only been provided by simulations, 10,11,24 but has also been verified in drop-tube experiments of Fe-Sn alloy. 11 At a temperature T 3 where the supersaturation reaches the critical value, the rings will break up into isolated globules, as displayed in Fig.…”

“…It was demonstrated by simulation that surface segregation before decomposition of the bulk liquid phase is favored thermodynamically. 10 Once a Bi-rich thin layer is formed on the surface, a coupled layer depleted in Bi emerges in its neighborhood, producing a concentration fluctuation along the radial direction of the droplet, as shown in Fig. 5b.…”

“…It was shown that the solidified microstructure of immiscible alloys depends strongly on many factors, including nucleation, diffusion, collision, Stokes and Marangoni motions, and wettability. 4,8 So, the formation of the core-shell morphology via phase separation is rather complicated; for example, many different kinds of morphology such as dual-layer, triple-layer, or even multilayer types have been reported, 5,[9][10][11][12][13] which cannot be attributed simply to Marangoni motion or the volume fraction of the minor phase. Interestingly, a more recent study by phase-field simulation revealed that several factors including spinodal decomposition, fluid flow, globule collisions, and Marangoni motion are strongly coupled, and their contributions to the evolution of microstructure change as the process proceeds.…”

Phase field simulation of monotectic transformation for liquid Ni-Cu-Pb alloys

“…The extended Model H simulates the phase separation of the binary alloy [18] . In the light of the different systems, the phenomenological model introduced into the modified item describes the evolutionary characteristics of alloy melt.…”

Understanding atomic-scale phase separation of liquid Fe-Cu alloy

“…The phase-separation processes in Fe-Cu and Ni-Cu-Pb alloys have been simulated by field methods. It has been shown that the Cu-or Pb-rich phase always forms the outer layer because of surface segregation, whereas internal microstructural evolution is mainly controlled by Marangoni convection [14,15]. In contrast, much work remains to be done in understanding atomic-scale phase separation.…”